Robert L. Terpstra

Sn-Ag-Cu solders and solder joints: Alloy development, microstructure, and properties

Slow cooling of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens made by hand soldering simulated reflow in surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shearstrength testing, using Sn-3.5Ag (wt.%) as a baseline. Minor substitutions of either cobalt or iron for copper in Sn-3.7Ag-0.9Cu refined the joint matrix microstructure, modified the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and increased the shear strength. At elevated (150°C) temperature, no significant difference in shear strength was found in all of the alloys studied. Ambient temperature shear strength was reduced by largescale tin dendrites in the joint microstructure, especially by the coarse dendrites in solute poor Sn-Ag-Cu.

Progress toward gas atomization processing with increased uniformity and control

Abstract Many advanced technologies based on particulate materials demand the availability of fine spherical powders or spherical powders of a narrow particle size class. Generally, high-pressure gas atomization (HPGA) is a close-coupled discrete jet atomization method and is one of the most effective methods of producing such powders. Development of HPGA nozzles with discrete jets resembling convergent–divergent (C–D) rocket nozzle designs, instead of the previous cylindrical jets, was conducted to increase atomization efficiency and uniformity and to reduce the required gas supply pressures. Results of compressible gas flow measurements on both types of HPGA nozzles revealed a steadily increasing trend of gas mass flow with gas supply pressure and a positive deviation from isentropic behavior that increases for increasing supply pressure. This has been attributed to an insufficient volume in the atomization nozzle gas manifold that experiences enhanced expansion cooling at increasing pressures. In experiments on 316L stainless steel, the atomization efficiency of the HPGA nozzle with C–D jets was higher than that of the HPGA nozzle with cylindrical jets, reflecting a lower gas/metal mass flow ratio. In other words, while the powder size distributions were nearly the same for all of the HPGA experiments, the HPGA nozzle with C–D jets utilized atomization gas with a significantly reduced operating pressure and mass flow rate.